1. Field of the Invention
Aspects of the present invention relate to a liquid crystal display (LCD) with a built-in touch screen, and more particularly, to an LCD with a built-in touch screen of a photosensor type, which has an operational reliability improved by combining a capacitance type with the photosensor type.
2. Description of the Related Art
A touch screen is provided on a front face of an image display device so that a user can select an instruction content displayed on a screen of the image display device using a user's hand or an object. The touch screen is in direct contact with the user's hand or the object. The touch screen senses a contact position, and the image display device receives a content instructed at the contact position as an input signal so as to be driven based on the input signal.
An image display device having a touch screen does not require a separate input device (such as a keyboard or mouse) connected to the image display device to operate. Hence, its application fields have been gradually extended. Recently, touch screens have been widely used in liquid crystal displays (LCDs). An LCD having a touch screen includes an LCD panel to display images and a touch screen panel positioned over the LCD panel to sense position information through a touch input from a user. At this time, a frame or adhesive is used to allow the touch screen panel to be positioned over the LCD panel, in which an air layer is formed between the LCD panel and the touch screen panel. In this case, a layer having a different refractive index from those of the LCD panel and the touch screen panel is formed between the LCD panel and the touch screen panel, and therefore, the optical characteristic of the entire LCD is deteriorated. Furthermore, since a separate touch screen panel is manufactured and then attached to the LCD panel, manufacturing cost is raised, and the thickness of the LCD is increased. Therefore, to solve such problems, an LCD with a built-in touch screen implemented by integrating a touch screen panel and an LCD panel is required.
Meanwhile, a photosensor or the like may be used to sense position information through a touch input from a user. In this case, a touch position is sensed by distinguish a portion covered by a contact object (such as a finger or touch stick) from a portion onto which external light is incident through the photosensor.
However, in the case of the touch screen of a photosensor type, if the illuminance of external light is low, a difference in illuminance between a portion onto which the external light is incident through a photosensor and a portion covered by a contact object is slight when a touch occurs. Therefore, the recognition rate of the touch may be lowered. On the other hand, if the illuminance of the external light is high, the portion onto which the external light is incident and the portion covered by the contact object are almost recognized as black and white when the touch occurs. Hence, the photosensor may erroneously recognize a shadow as a touch even when a user's finger or object is not touching a touch screen panel, but is located over the touch screen panel. Therefore, operational reliability may be lowered. Moreover, if the illumination of the external light is high, the photosensor may recognize a shadow as a multi-touch even when a multi-touch does not occur. For example, when the touch screen panel is touched by one finger and another finger is located over the touch screen panel, the photosensor may erroneously determine that the touch screen panel is touched by both of the fingers, and therefore, may be erroneously recognize a multi-touch.
Accordingly, there is a need to secure the operational reliability of the touch screen by precisely recognizing a touch event.